Good introduction to Nix. I wish I would have had this when I started. But what do they mean with “works forever”? What if the Git repo or Web server of the pr

Good introduction to Nix. I wish I would have had this when I started. But what do they mean with “works forever”? What if the Git repo or Web server of the project is gone in several years? How do you reproduce it? Does niv include a local source code mirror?